An embodiment of an illumination device of the kind set forth is known from U.S. Pat. No. 7,709,811. That document discloses an illumination device comprising a blue LED (e.g. GaN or InGaN) light source, an internal optical element, a wavelength converting material, and an external optical element (e.g. a plastic or glass lens). The wavelength converting material (e.g. an organic dye or inorganic phosphor) is applied to a side of the internal element facing away from the LED. The internal optical element is a rectangular or pyramid shaped prism and serves to direct primary light emitted by the LED to the wavelength converting material. Moreover, it serves to redirect secondary light emitted by the wavelength converting material in the backward direction (i.e. towards the LED) to a forward direction (i.e. away from the LED). The external optical element serves to define an application specific illumination distribution consisting of a mixture of the primary and secondary light.
Devices as disclosed by U.S. Pat. No. 7,709,811 exhibit several difficulties limiting their usefulness, such as heat management issues, efficiency issues, and emission directionality issues.
For instance, many illumination applications prescribe LED based systems providing power levels on the order of a few Watts. When concentrating light with such power levels in a relative small volume of phosphor material, the Stokes losses inherent to the wavelength conversion process result in high local heat dissipation. With a typical conductivity of 0.1-10 WK-1m-1 common to most phosphorous materials, heat transportation becomes a limiting factor at a typically applied thickness (˜100 μm) of the phosphor layer necessary for realizing sufficient absorption of the exciting primary wavelength light. This results in alleviated temperature levels of the phosphor which can easily exceed 200-300° C. At such levels, the conversion efficiency of the phosphors drops significantly, potentially resulting in additional power losses and uncontrolled further heating.
Moreover, the overall efficiency of such illumination devices depends on the efficiency of the excitation and emission processes in the wavelength converting material. The excitation efficiency depends on the absorption strength of the phosphor at the primary wavelength light emitted by the LED. The emission efficiency is influenced by both the extent to which the absorbed energy (i.e. primary wavelength light) is converted into emitted energy (i.e. secondary wavelength light) and the extent to which this emitted energy is coupled out of the device in a forward direction. With respect to the absorption efficiency, many wavelength converting materials exhibit a relative low absorption coefficient (typically 10-100 cm-1 upon excitation in the 400-480 nm range). This implies that a 100-1000 μm thick layer of wavelength converting material is required for sufficient, or even complete, absorption of the excitation radiation. Such relatively large thicknesses may lead to an extended size of the light emitting area, especially when used in combination with laser light sources, and thus to a limited use of such devices as low étendue light sources in f.i. projection applications as beamers or car head lights.
Furthermore, a flat emission surface of the wavelength converting material gives rise to a Lambertian emission profile. While beam shaping optical elements are known to be useful to realize the application specific illumination distribution, these optical elements are usually bulky, need precise alignment with the LED and/or wavelength converting material, and are typically based on weakly dispersing materials (e.g. glass, plastics) which do not allow different beam shaping and beam directing of different light colors.